This invention relates to bone screws used in the fixation of artificial prosthesis or autogenous grafts within the body and, in particular, to the short and long-term mechanical fixation of such components in their application to joint reconstruction. More specifically, the invention is directed to a screw for securing an acetabular cup of an acetabular prosthesis or for a bone-tendon autograft in an anterior cruciate ligament (ACL) repair procedure.
One of the critical problems with the use of artificial joints to replace diseased or traumatized joints is acquiring a reliable attachment of the prosthesis to the bone in order to achieve both initial and long-term fixation. The most widely used method to secure a prosthesis to bone and provide fixation involves the use of bone cement. While this approach in general provides a strong initial bond, it has been found that the bone cement process is inadequate and undesirable since, through a multitude of possible mechanisms upon which no consensus has been reached, the bone-cement interface eventually breaks down. In situ, the cement is often the first part of the prosthetic reconstruction to fail during use.
Current attempts to increase prosthesis function have focused on establishing long-term fixation between the implant and the surrounding bone. The technology that has been developed to meet this need involves the provision of prosthetic implants with porous-coated areas designed to accept bone ingrowth. It has been found that in order to optimize the potential of this porous-coated technology, it is imperative to maximize the porous-coated area of the prosthesis in contact with the adjacent bone. However, a substantial number of implants continue to fail over time. Lack of significant bone ingrowth is a leading cause of such failures and this has been attributed in part to inadequate initial fixation and micromotion between the implant and the bone.
Numerous attempts to improve initial fixation and prevent micromotion have been developed. For instance, U.S. Pat. No. 4,883,491 discusses the use of an acetabular cup with an outer surface which includes screw threads. The screw threads provide for the cup to be attached directly into the acetabular socket through the application of torque to the cup at the time of implantation. This is disclosed as providing strong initial fixation of the implant to the bone. To provide long-term fixation to the bone, the acetabular cup disclosed in U.S. Pat. No. 4,883,491 includes porous-coated columns in an alternating pattern with the screw threads on the surface of the cup. One drawback of such an implant is that its installation requires removal of significant amounts of subchondral bone. Recent studies have shown that removal of large amounts of subchondral bone results in loss of the subchondral plate, weakening of the pelvis and inadequate long-term fixation.
Others have attempted to provide initial and long-term fixation through the combination of a porous-coating on the implant, and bone screws to provide for initial fixation. Current screws used to secure a cementless acetabular prosthesis in total hip replacements consist of a simple spiral shank screw. The screw is intended to provide initial stability to the implant, while long-term stability is dependent both on the initial stability of the screws and subsequent bone ingrowth into the porous-coated area on the external surface of the acetabular cup.
Currently, upwards of 40% of all acetabular implants fail at ten years, with the incidence of failure increasing substantially thereafter. Even with recent advances, it has been noted that the primary reason for failure is loosening of the acetabular implant. While the cementless implant has sufficient initial stability through support from the screws, the screws cannot maintain the stability required to effectively initiate bone ingrowth since they provide no mechanism for increasing stability once inserted into the pelvis. As a result, the stresses that are continually transferred to the screws act to slowly work the screws loose and subsequently decrease implant support. The increasing ineffectiveness of the screws then transfers more stress to the acetabular cup itself, which results in micromotion of the prosthesis followed by the accumulation of prosthetic wear debris and an inflammatory immune reaction that destroys the bone-implant boundary. The result is concluded to be complete prosthetic failure.
Acetabular cups used in conjunction with screws are often provided with porous-coated fixation pegs and fins. Although these devices provide good initial mechanical fixation, many problems remain. For instance, acetabular prostheses that have been designed to date have failed to prevent the main mode of acetabular failure, where the acetabular prosthesis sinks into the bony pelvis. In addition, no attempt has been made to maintain the initial fixation provided by the bone screw or threaded acetabular cup for the purpose of long-term fixation. Thus, while threaded acetabular cups, and acetabular cups secured by bone screws, provide strong initial fixation, the literature has shown that resorption occurs around the screw threads after six to eight weeks. As a result, the screws are rendered less effective after this period of time and some of the stress initially carried by these screws or acetabular threads is transferred to the prosthesis.
The continuous transfer of stresses to the prosthetic implant alters the stress shielding characteristics of the implant, and creates a progressive syndrome of stress shielding. This syndrome results in a continuously changing pattern of bone resorption and hypertrophy, leading to instability of the implant. In addition, the continually changing pattern of resorption and hypertrophy, as well as the transfer of stresses to the prosthesis, results in micromotion of the whole assembly.
Reconstruction of the hip joint with an artificial prosthesis is not the only surgical reconstructive procedure plagued with problems of long-term viability. Anterior cruciate ligament repair procedures have utilized numerous techniques ranging from extraosseosis fixation of the autologous bone-tendon graft with screws and staples to the use of completely artificial ligaments in an attempt to provide increased long-term viability of the reconstructed joint (knee). Because problems with artificial ligament development have not yet been adequately worked out, and for a multitude of other reasons based on surgical preference, arthroscopic compatability and such, bone tendon autografts using patellar tendons have become the graft of choice for anterior cruciate ligament reconstruction.
Secure immediate and long-term fixation of the graft is critical to the success of the procedure. This will be better understood with an understanding of the basic method in which ACL repair procedures are performed. A bone plug is taken from both the patella and the tibial tuberosity which are connected to each other by the patellar ligament. A bone-tendon autograft involves securing a patellar bone plug, patellar ligament and tibial bone plug. To perform the ACL procedure, two holes are drilled, one in the distal femur and one in the proximal tibia. The bone-tendon autograft is then inserted into the holes such that the tibial bone plug is in the tibial hole and the patellar bone plug is in the femoral hole. The patellar ligament thus assumes a position similar to the natural position of the ACL.
The bone plugs are commonly secured within their holes by an interference fit with a Kurosaka screw. Optimally, the Kurosaka screw maintains adequate fixation of the graft such that bone plug to bone fixation can secure the graft permanently. However, problems have arisen with this procedure in that resorption around the Kurosaka screw and the potential for stress shielding and subsequent bone remodeling cause a progressive slipping of the screw in the face of the constant pulling force of the tendon against the screw. Eventually the tendon becomes sufficiently lax to cause complete failure of the reconstruction.